The expression 'to be in it for the long haul' means to do something until it is finished or complete, to do something for a long period of time even though it may be difficult or require patience.

1. I realise that getting my degree will be a lengthy, tough process, but I'm in it for the long haul. I will stick with it until the end.
2. Marriage is something you are in for the long haul, you can't just walk away when things get tough.

1. When I invest in a business I'm not just looking to make a quick profit; I'm in it for the long haul. 
2. Starting a business requires a lot of work and sacrifice, you have to be in it for the long haul.